Public power has been justified by resorting to two different kinds of legitimation: the one coming from above, the other emerging from the governed. While the legitimation «from above» implies that those who are vested with executive power are qualified to their function because of their allegedly higher competences, the «bottom-up» legitimacy always presupposes that only the citizens can properly decide on their destiny. After giving a brief account of how both legitimation strategies have developed in the history of political ideas, the author focuses his attention on the theories regarding the legitimacy of public power in the European Union. Indeed, both strands of legitimation of public power are here represented with original proposals, according to the specificity of the supranational condition. But even more interesting is that the research into the characteristics of supranational integration has been one of the most significant fields in which the legitimation «from above» has reappeared in Western thought after a rather long period of marginality, now taking the shape of a technocratic justification. In the main section of the article, the reasons in favour of a democratic «bottom-up» legitimation of the European public power are analyzed first, then those which recur to the so-called «output legitimacy» —in other words to technocratic arguments. The last section of the contribution is dedicated to an overall assessment of the different positions.